DNA Gyrase: We have continued our studies of DNA gyrase, the enzyme that is responsible for DNA supercoiling in prokaryotic cells. The specific sites of DNA breakage caused by the enzyme in the presence of oxolinic acid have been characterized. The two DNA strands are always broken four bases apart, with the 5'-ended single strand protruding and blocked by the protein. We have also cloned the genes coding for the subunits of DNA gyrase, which allows to make the large quantities of enzyme needed for mechanistic studies.